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EC number: 204-625-1 | CAS number: 123-41-1
- Life Cycle description
- Uses advised against
- Endpoint summary
- Appearance / physical state / colour
- Melting point / freezing point
- Boiling point
- Density
- Particle size distribution (Granulometry)
- Vapour pressure
- Partition coefficient
- Water solubility
- Solubility in organic solvents / fat solubility
- Surface tension
- Flash point
- Auto flammability
- Flammability
- Explosiveness
- Oxidising properties
- Oxidation reduction potential
- Stability in organic solvents and identity of relevant degradation products
- Storage stability and reactivity towards container material
- Stability: thermal, sunlight, metals
- pH
- Dissociation constant
- Viscosity
- Additional physico-chemical information
- Additional physico-chemical properties of nanomaterials
- Nanomaterial agglomeration / aggregation
- Nanomaterial crystalline phase
- Nanomaterial crystallite and grain size
- Nanomaterial aspect ratio / shape
- Nanomaterial specific surface area
- Nanomaterial Zeta potential
- Nanomaterial surface chemistry
- Nanomaterial dustiness
- Nanomaterial porosity
- Nanomaterial pour density
- Nanomaterial photocatalytic activity
- Nanomaterial radical formation potential
- Nanomaterial catalytic activity
- Endpoint summary
- Stability
- Biodegradation
- Bioaccumulation
- Transport and distribution
- Environmental data
- Additional information on environmental fate and behaviour
- Ecotoxicological Summary
- Aquatic toxicity
- Endpoint summary
- Short-term toxicity to fish
- Long-term toxicity to fish
- Short-term toxicity to aquatic invertebrates
- Long-term toxicity to aquatic invertebrates
- Toxicity to aquatic algae and cyanobacteria
- Toxicity to aquatic plants other than algae
- Toxicity to microorganisms
- Endocrine disrupter testing in aquatic vertebrates – in vivo
- Toxicity to other aquatic organisms
- Sediment toxicity
- Terrestrial toxicity
- Biological effects monitoring
- Biotransformation and kinetics
- Additional ecotoxological information
- Toxicological Summary
- Toxicokinetics, metabolism and distribution
- Acute Toxicity
- Irritation / corrosion
- Sensitisation
- Repeated dose toxicity
- Genetic toxicity
- Carcinogenicity
- Toxicity to reproduction
- Specific investigations
- Exposure related observations in humans
- Toxic effects on livestock and pets
- Additional toxicological data
Dissociation constant
Administrative data
Link to relevant study record(s)
- Endpoint:
- dissociation constant
- Type of information:
- experimental study
- Adequacy of study:
- weight of evidence
- Study period:
- 2013-03-06 to 2013-03-07
- Reliability:
- 1 (reliable without restriction)
- Rationale for reliability incl. deficiencies:
- other: Well documented guideline study (OECD 112) without restrictions.
- Reason / purpose for cross-reference:
- reference to same study
- Reason / purpose for cross-reference:
- reference to same study
- Reason / purpose for cross-reference:
- reference to same study
- Reason / purpose for cross-reference:
- reference to same study
- Reason / purpose for cross-reference:
- reference to same study
- Reason / purpose for cross-reference:
- reference to same study
- Reason / purpose for cross-reference:
- reference to same study
- Qualifier:
- according to guideline
- Guideline:
- OECD Guideline 112 (Dissociation Constants in Water)
- Deviations:
- no
- GLP compliance:
- no
- Dissociating properties:
- yes
- No.:
- #1
- pKa:
- 11.2
- Temp.:
- 21.5 °C
- Conclusions:
- The dissociation constant of Choline hydroxide was determined in a OECD 112 guideline study without deviations by titration to be pKa = 11.2 at 21.5 °C. So in general, the results are considered as reliable. However, the results obtained show a strong concentration dependence causing significant variation in the individual pKa results obtained and therefore the calculation of the ideal pKa by extrapolation to zero concentration was performed. As the true pKa value assumes infinite dilution i.e. is a measure of the activity of hydrogen ions, and therefore should not be influenced by surrounding ions, the pKa value to be reported will be that obtained using 0.1M HCl and therefore calculated using the lowest concentrations.
Since due to the alkaline properties of choline base (pH = 14.9 of a 45 % solution), the pKb value is the more relevant one and can be assumed to be pKb = 14.1 – pKa = 2.9, indicating dissociating properties. - Executive summary:
The dissociation constant of Choline hydroxide was determined in a OECD 112 guideline study by titration to be pKa = 11.2 at 21.5 °C.
Reference
Results (0.5M sulphuric acid)
As often happens when titrating hydroxides, two end points were detected with a small difference between the two. For the purpose of determination of the dissociation constant, a single equivalence point, to be used to determine the appropriate bracketing measurements, was used, at the mid-point of the curve between the two end points.
Table 1. End Point Titre
Wt. of test substance (g) | Titre (mL) |
0.9918 | 3.7355 |
1.6384 | 6.1741 |
2.4684 | 9.3115 |
Calculation of pKa
The pKa was given by the pH value corresponding to half the volume of titre required to attain the equivalence point. This was calculated in triplicate from the titre/pH value tables generated by the auto-titrator by using the pH of the volumes bracketing the half volume of the end point (4 data points) to prepare a graph and then using the graph to determine the pH at half volume.
Table 2. Raw data for calculation of pKa
Test | End Point Volume (mL) | Half Volume (mL) | Bracketing Volume (mL) | pH |
1 | 3.7355
| 1.8677 | 1.1170 | 12.532 |
1.7070 | 12.419 | |||
2.2230 | 12.281 | |||
2.6430 | 12.131 | |||
2 | 6.1741
| 3.0870 | 1.7960 | 12.668 |
2.4660 | 12.585 | |||
3.0880 | 12.506 | |||
3.7030 | 12.396 | |||
3 | 9.3115 | 4.6557 | 3.9480 | 12.739 |
4.6270 | 12.675 | |||
5.2830 | 12.606 | |||
5.9260 | 12.519 |
Calculation of pKa for Test 1: Y= -0.2606 x 1.8677 + 12.842 = 12.36
Calculation of pKa for Test 2: Y= -0.1409 x 3.0870 + 12.928 = 12.49
Calculation of pKa for Test 3: Y= -0.1105 x 4.6557 + 13.181 = 12.67
Table 3. calculated pKa values
Test | Weight sample (g) | pKa measured |
1 | 0.9918 | 12.36 |
2 | 1.6384 | 12.49 |
3 | 2.4684 | 12.67 |
To calculate the pKa at ideal (zero) weight, a graph is plotted of the pKa values determined versus the weight of test substance and extrapolated to the intercept on the x-axis: extrapolated to zero weight = 12.15, reported as 12.2.
The individual results do vary considerably in accordance with the weight taken indicating a significant concentration dependence. Because of this, it was decided to perform a further triplicate test using lower weights of test substance and using a titrant of 0.1M hydrochloric acid.
Results (0.1M hydrochloric acid)
Again two end points were detected with a small difference between the two. For the purpose of determination of the dissociation constant, a single equivalence point, to be used to determine the appropriate bracketing measurements, was used, at the mid-point of the curve between the two end points.
Table 4: End Point Titre
Wt. of test substance (g) | Titre (mL) |
0.0760 | 2.7602 |
0.1600 | 5.8044 |
0.2934 | 10.5923 |
Calculation of pKa
The pKa was given by the pH value corresponding to half the volume of titre required to attain the equivalence point. This was calculated in triplicate from the titre/pH value tables generated by the auto-titrator by using the pH of the volumes bracketing the half volume of the end point (4 data points) to prepare a graph and then using the graph to determine the pH at half volume.
Table 5. Raw data for calculation of pKa
Test | End Point Volume (mL) | Half Volume (mL) | Bracketing Volume (mL) | pH |
1 | 2.7602
| 1.3801 | 0.7380 | 11.493 |
1.2800 | 11.358 | |||
1.7740 | 11.178 | |||
2.0920 | 10.998 | |||
2 | 5.8044
| 2.9022 | 2.0660 | 11.756 |
2.7270 | 11.641 | |||
3.2000 | 11.569 | |||
3.7770 | 11.472 | |||
3 | 10.5923 | 5.2961 | 3.9580 | 11.919 |
4.6620 | 11.867 | |||
5.3540 | 11.811 | |||
6.0310 | 11.746 |
Calculation of pKa for Test 1: Y= -0.3573 x 1.3801 + 11.782 = 11.29
Calculation of pKa for Test 2: Y= -0.1651 x 2.9022 + 12.095 = 11.62
Calculation of pKa for Test 3: Y= -0.0832 x 5.2961 + 12.252 = 11.81
Table 6: calculated pKa values
Test | Weight sample (g) | pKa measured |
1 | 0.0760 | 11.29 |
2 | 0.1600 | 11.62 |
3 | 0.2934 | 11.81 |
Result extrapolated to zero weight = 11.17, reported as 11.2.
Summary of Results
The results obtained show a strong concentration dependence causing significant variation in the individual pKa results obtained and therefore the calculation of the ideal pKa by extrapolation to zero concentration. As the true pKa value assumes infinite dilution i.e. is a measure of the activity of hydrogen ions, and therefore should not be influenced by surrounding ions, the pKa value to be reported will be that obtained using 0.1M HCl and therefore calculated using the lowest concentrations.
Dissociation constant:11.2 at 21.5 °C
Description of key information
In accordance with section 1 of REACH Annex XI, the dissociation constant study does not need to be conducted as the substance is a salt. Salts are reaction products of acids and bases that retain their ionic character. So salts are never undissociated. The dissociation constant is only relevant for substances with a acid or basic character.
The dissociation constant of a 45 % aqueous solution has been determined: 11.2 at 21.5 °C (OECD Guideline 112 (Dissociation Constants in Water))
Key value for chemical safety assessment
- pKa at 20°C:
- 11.2
Additional information
First of all the dissociation constant study does not need to be conducted as the substance is a salt. Salts are reaction products of acids and bases that retain their ionic character. So salts are never undissociated. The dissociation constant is only relevant for substances with a acid or basic character.
Nevertheless choline hydroxide has an ionic substance and one of the ions contains an alcohol group which typically is able to dissociate. Choline hydroxide is a quaternary amine salt, it can be easily concluded that it dissociates in water into the corresponding positively charged quaternary hydroxyl alkylammonium ion and the negatively charged respective anion (OH-).
This is because typically salts (ionic bonding) are electrovalent substance. Electrovalent substances are made up of ions in the solid state. The oppositely charged ions are held together by strong electrostatic (coulombic) force of attraction. Due to these forces the ions cannot move. When these substances are dissolved in water, the ions free themselves from this binding. Thus the break up of an electrovalent compound into free mobile ions when dissolved in water or when melted, is called electrolytic dissociation. This is a 100 % dissociation. Nevertheless the quaternary ammonium salt has an alcohol group which is from a chemically point of view able to dissociate. This dissociation from to alcohol into a proton and the hydroxyl-molecule is calculated as mentioned above and can be considered as the dissociation constant beside the electrolytic dissociation.
The available determination of the dissociation constant of choline hydroxide (45 % aqueous solution) according to OECD 112 Guideline study without deviations by titration will be used for the purpose a PNEC calculation according to the equilibrium partitioning method. The pKa value was reported to be pKa = 11.2 at 21.5 °C. In general, the results are considered as reliable. However, the results obtained show a strong concentration dependence causing significant variation in the individual pKa results obtained and therefore the calculation of the ideal pKa by extrapolation to zero concentration was performed. As the true pKa value assumes infinite dilution i.e. is a measure of the activity of hydrogen ions, and therefore should not be influenced by surrounding ions, the pKa value to be reported will be that obtained using 0.1M HCl and therefore calculated using the lowest concentrations. Since due to the alkaline properties of choline base (pH = 14.9 of a 45 % solution), the pKb value is the more relevant one and can be assumed to be pKb = 14.1 – pKa = 2.9, indicating dissociating properties.
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